function f=langmuir(a,x);
%SS=a(5)*exp(-((x-a(6))/abs(a(7))).^2);
%SS=exp(a(5)*tanh((x+a(6))/abs(a(7))));
f=exp(a(1)*tanh((x+a(2))/abs(a(3))))-a(4);%+SS;
%f=-a(1)*(1-exp((x-a(2))/abs(a(3))));